The number of cellular phone users in the U.S. is now estimated at about 17 million and continues to grow at a rate exceeding 20 percent per year. Most of this growth in use has occurred in and around cities and towns with populations of 20,000 or more and has caused saturation of the channels available for cellular communication in many of these urban areas. Cellular service zones, referred to herein as "cellzones" or "cells" for convenience, servicing about 800 cellular telephone communications have decreased in size from several miles in diameter to a few hundred meters in diameter as the density of cellphone users has increased apace in urban and suburban areas.
A cellphone subscriber will typically register use of a cellphone in a home cellzone and will temporarily re-register in one or more nearby cellzones if the subscriber makes or receives cellphone calls in these other cellzones. However, even where the cellphone subscriber is already registered in one-or-more nearby cellzones, the operator of any cellzone will typically be unaware of the presence of this subscriber, unless this subscriber has either recently requested cellular service in this zone or has somehow notified the operator that the subscriber is currently present in this zone. Knowledge of the number of cellphone subscribers currently in a given cellzone, who are not all active users of the cellular phone service at any given moment, may allow the service operator to rationally plan its present provision, and future expansion, of cellular service to accommodate the increasing demands.
Typically, a geographic region is divided into a plurality of polygonal cells having determinable diameters, and each cell is allocated a mutually exclusive subset S1, S2, S3, etc. of cellphone frequencies, as illustrated in FIG. 7. If the cellphone signal strengths and cellular service provider (MTSO) signal strengths are chosen carefully, a first cellzone located at least three cell diameters away from a second cellzone may use the same set of cellular frequencies as does the first cellzone, and little or no cellular signal interference will result from this re-use of cellular signal frequencies. About 333 cellular frequencies are available for each MTSO, with one pair being used for each cellular phone call or for supervisory signaling, so that an average of about 333.div.7.apprxeq.47 cellular frequency pairs are available for use in cellular phone calls and supervisory signaling per cellzone. Cellular phone protocols and allocation of frequency pairs are discussed by D. M. Balston and R. C. V. Macario, Cellular Radio Systems, Artech House, Boston, 1993, pp. 3-72, by J. D. Gibson (ed.), The Mobile Communications Handbook, CRC Press, 1996, Boca Raton, pp. 242-288 and 319-332, and by W. C. Y. Lee in Mobile Cellular Telecommunications, McGraw Hill, Second Edition, 1995, pp. 53-70 and 257-280.
When responsibility for servicing a given cellphone is handed off from a first cell site to a second cell site, the second cell site may not have presently have an available cellular frequency pair that can be used to service the given cellphone and handoff is unavailable. In this instance, cellular service for that cellphone is unceremoniously dropped. Further, the cellphone may find itself in a signal strength "hole" or gap within a formally defined cell, where the strength of a cellular signal received from the responsible cell site is too weak to support a cellular connection with reasonable signal quality, and signals received from cell sites located in adjacent cells also have insufficient strength. In these two situations, discussed by Lee, op cit, pp. 283-305, handoff may be unavailable. This may be tolerable where the given cellphone is initiating or receiving a cellphone call and the voice or other exchange has not yet begun; the initiating cellphone merely re-initiates the cellphone call. However, dropping cellphone service in the middle of a conversation may have serious consequences for the parties to the conversation, especially if the conversation includes transmission of data. Lojack Corp. of Dedham, Mass. has demonstrated a vehicle location system using triangulation of radio signals transmitted from the vehicle by a large number of receivers that are strategically positioned around a community. The vehicle transmitter is remotely activated by a signal broadcast from elsewhere.
Vehicle and person location techniques using location determination systems, including cellular phone systems, are disclosed in several U.S. patents that rely upon Global Positioning System (GPS) signals or selected radiowave signals to determine the user's present location.
Cellular phone handoff techniques, relying mostly upon analysis of changing signal strength, response signal time delay or assigned call priority, are disclosed in several recent U.S. patents, including U.S. Pat. No. 5,483,664, issued to Moritz et al, U.S. Pat. No. 5,546,445, issued to Dennison et al, U.S. Pat. No. 5,551,058 and U.S. Pat. No. 5,551,059, issued to Hutcheson et al, U.S. Pat. No. 5,557,657, issued to Barnett, U.S. Pat. No. 5,640,676, issued to Gamcarz et al, and U.S. Pat. No. 5,649,291, issued to Tayloe.
These patents disclose use of GPS or radiowave technology for determination of location or orientation of a person or vehicle continuously or at discrete times, usually without regard to which of several geographically defined zones the person or vehicle may last have entered. What is needed is a system that automatically tracks the movement of a vehicle or person, such as a cellphone subscriber, who moves from one cellzone to another, irrespective of whether a given cellphone subscriber is actively using cellular phone service as a boundary is crossed between two adjacent cellzones. Preferably, this system should accurately determine which cellzone a given subscriber currently moves in and should be flexible enough to cover cellzones of arbitrary and changeable sizes, shapes and densities.
What is further needed is a system that uses its knowledge of the location vector and velocity vector of a cellphone subscriber, who is currently engaged in a cellular telephone conversation, (1) to advise the subscriber of an approaching crossing of a cell boundary, (2) to advise the subscriber whether an unused cellular frequency pair is (likely to be) available in a "handoff" cell into which the subscriber is likely to pass, (3) to advise the subscriber of an anticipated or extant decrease in signal strength that seriously compromises the system's ability to maintain a cellular connection with the required signal quality, (4) to allow the subscriber to take appropriate action within the present cellzone in order to avoid dropping the cellphone connection and (5) to provide cellphone location reporting to facilitate decision making by a cellular service provider.